The invention relates to image intensifier tubes, and more particularly to a magnetically-focused image intensifier tube having an internal alkali baffle which permits the external location of the alkali vapor sources used to form the photoemissive cathode of the tube.
An image tube is a type of tube which employs a photoemissive cathode or photocathode which is sensitive to radiation across a particular wavelength region of the electromagnetic spectrum. When exposed to such radiation, electrons are emitted from the photocathode and are caused to travel to a phosphor-coated anode or screen where they strike the phosphor giving off light. Magnetically-focused image intensifier tubes combine an electrostatic accelerating field with an axial magnetic-focusing field provided either by a solenoid or a permanent magnet. The uniform magnetic field provides good resolution over the entire phosphor screen with minimum image distortion.
In prior art magnetically-focused image intensifier tubes, the alkali vapor sources for forming the photocathode were located within the evacuated envelope structure. One of the drawbacks of having the alkali vapor sources internal to the envelope was that photocathodes formed in such tubes often showed variations in spectral sensitivity which, in some instances, were extreme. For example, some tubes having the well-known S-20 multialkali photocathode were fabricated in which the spectral sensitivity at 420 nanometers often varied by only a few percent; however, the spectral sensitivity variation at 800 nanometers in the same tubes often approached fifty percent. It is believed that nonuniform spectral sensitivity at various wavelengths is caused primarily by nonuniform sodium distribution within the photocathode.
The formation of photocathodes within image intensifier tubes by means of an "external" evaporation process is described in U.S. Pat. No. 2,244,720 issued to Massa et al. on June 10, 1941. In U.S. Pat. No. 2,752,519 issued to Ruedy, on June 26, 1956, a connected external chamber houses a substance that is inserted into the main envelope of the tube to form the photocathode. After forming the photocathode, the holder is retracted into the chamber. Both the Massa et al. and the Ruedy patents show an internally-disposed apertured element adjacent to the external alkali chamber that permits the alkali materials from the chamber to pass directly into the vacuum enclosure. In the Massa et al. structure, the apertured element adjacent to the external alkali source does nothing to provide a uniform distribution of alkali material and causes a nonuniform spectral variation in the photocathode produced thereby.
An improved structure for externally processing a photocathode is shown in U.S. Pat. No. 3,894,258 issued to Butterwick on July 8, 1975. The Butterwick structure shows a baffle chamber adjacent to the anode element of a proximity-focused image tube. A drawback of the Butterwick structure is that the alkali material which evolves from the baffle chamber through a space adjacent to the phosphor screen tends to contaminate the screen during the formation of the photocathode. Only by providing a suitably thick aluminized layer over the phosphor screen is alkali contamination prevented. However, a thick aluminum layer reduces the energy of the electrons which reach the screen and thereby lowers the gain of the tube.
U.S. Pat. No. 4,157,484 issued to Stowe et al. on June 5, 1979 and U.S. Pat. No. 4,198,106 issued to Stowe et al. on Apr. 15, 1980, show a typical electrostatically-focused image tube having an external alkali processing system and a baffle structure incorporated therein. In the image tube of the Stowe et al. patents, the baffle system is located adjacent to the photocathode and comprises a metal vapor shield which forms a part of the focusing structure for the electrons leaving the photocathode. The Stowe et al. baffle structure is not applicable to magnetically-focused image intensifier tubes since the metallic baffle structure of the Stowe et al. patents would distort the uniform electrostatic accelerating field adjacent to the photocathode. The metal vapor shield also would dangerously increase the voltage gradients in a magnetically-focused image intensifier tube. Likewise, the baffle structure of the Butterwick patent, in which the baffle chamber is adjacent to the anode, is also unusable in a magnetically-focused image intensifier tube since the screen of a magnetically-focused image intensifier tube is exceptionally prone to alkali contamination especially in multistage structures where it is impossible to heat the screen to a higher temperature than the cathode as suggested in the Butterwick patent to minimize alkali contamination of the phosphor.